(a) Field of the Invention
This invention relates to a method of enhancing the growth of aquatic organisms and to structures created thereby and more particularly, it relates to the creation of artificial reefs, coastal defence structures and aquaculture facilities which consist of man-made and biologically produced materials, the latter having enhanced growth rates. In the case of aquaculture facilities, more particularly mariculture facilities, such as oyster-producing installations, the shellfish or other grown organism is harvested.
(b) Description of the Related Art
Sea water contains nine major elements: sodium, magnesium, calcium, potassium, strontium, chlorine, sulphur, bromine and carbon.
These elements comprise more than 99.9% of the total dissolved salts in the ocean (see Milliman et. al., Marine carbonates, Springer Verlag New York 1974; Sverdrup, et. al. The Oceans: Their Physics, Chemistry and General Biology, Prentiss-Hall, Inc., in New Jersey 1942; and Culkin and Goldberg in Volume 1, Chemical Oceanography, pp. 121-196, Academic Press, London 1965). The constancy of the ratios of the major elements throughout the oceans has long been well known (Dittmar, Challenger Reports, Physics and Chemistry, pp. 1-251, 1884).
In 1940 and 1947, G. C. Cox was issued U.S. Pat. Nos. 2,200,469 and 2,417,064, outlining methods of cathodic cleaning and protection of metallic surfaces submerged in seawater by means of a direct electrical current. During the cleaning process, a coating is also formed cathodically, consisting of magnesium and calcium salts (Eichoff and Shaw, Corrosion, No. 4, pp. 363-473, 1948). If these coatings are hard and continuous, they afford a considerable degree of corrosion protection to the enclosed metal (see Humber, Corrosion, No. 4, pp. 358-370, 1948, and Corrosion, Volume 4, No. 9, pp. 292-302, 1949).
Lower marine organisms utilize the minerals in solutions surrounding them to build structural formations. Mollusk shells, for example, are generally composed of calcium carbonate crystals enclosed in an organic matrix. A significant proportion of the soluble protein in the matrix is composed of a repeating sequence of aspartic acid separated by either glycine or serine (see Jope in Volume 26, Comprehensive Biochemistry, p. 749, Elsevier, Amsterdam, 1971). This sequence, comprising regular repeating negative charges, could bind Ca.sup.2+ ions and thus perform an important function in mineralization of the template (Weiner and Hood, Volume 190, Science, pp. 987-989, 1975).
The present inventors have studied the electrodeposition of materials from seawater over a considerable number of years (see U.S. Pat. Nos. 4,461,684, 4,440,605 and 4,246,075, all of Hilbertz).
In AMBIO--a Journal of the Human Environment, published by the Royal Swedish Academy of Sciences, Vol. 21 No. 2, April 1992, page 126-129, Hilbertz, surveyed existing resources for the operation of mineral accretion processes and discussed similarities in biogenic calcium carbonate deposition. Electrolytically precipitated limestone and material consisting of artificial limestone and hard coral skeletons were proposed as a sink for carbon and for use as building materials. It was postulated that solar-generated building material (SBM) could be used to stimulate reef and shell-building organisms, creating artificial reef communities. An illustration was included of an early experiment to graft hard coral onto SBM. A branch of elkhorn coral (Acropora palmata) was covered by a cathode of galvanized wire mesh. A single anode was placed nearby. Both electrodes were connected to a small photovoltaic cell above sea level, causing mineral accretion (MA) to envelop the coral branch. After completed accretion (15 days later) coral began to colonize the SBM, starting at the coral sided fringe of the cathode covered with MA. Similar investigations were in progress involving multiple specimens of several live coral species which were attached to MA surfaces without electricity and cathodes producing MA. The coral specimens were being monitored to determine the effects of electric fields, hydrogen evolution at the cathode, pH of the surrounding electrolyte, ion density in the vicinity of the cathode, and other factors.
The article was based on a small-scale trial using an existing coral and pointed out the need for further experimental observations. In particular, no comparison was made of coral growth rates as compared to other environments and no reasons were given to indicate that growth of coral or other organisms would in fact be stimulated by using the SBM substrate.
Schuhmacher et al in Bulletin of Marine Science, 55(2-3); 627-679, 1994 describe integrated electrochemical and biogenic deposition of hard material to provide a substrate for hard sea-bottom settlers. Brucite, aragonite and other materials derived from the ambient seawater are precipitated on a cathode of the shape desired. An iterative pattern of direct-current phases and dead phases permits the calcareous matter to be deposited in an integrated fashion by physical precipitation and by secretion by sessile organisms. During the dead phase the substrate is colonized by a diverse community of the type which occurs on natural hard substances. However the experiments showed that simultaneous electrochemical and biogenic deposition of hard material was not possible. A dead phase of some four weeks was necessary. The present inventors consider that the lack of success in achieving growth of the lime-secreting organisms during electrodeposition was due to the location of the anode as shown in FIG. 3 on page 674 of the reference. The placing of the anode inside a cylindrical or conical cathode would have the result that hydrochloric acid generated at the anode would pass the cathode and would adversely affect living organisms in the vicinity of the cathode.
It is an object of the present invention to provide artificial reefs which are superior to traditional artificial reefs like those consisting of old cars, sunken ships, used automobile tires and precast plastic and concrete modules or elements.
It is a further object of the invention to grow shore defence structures like groins and sea walls economically using biological building materials in connection with the mineral accretion process.
It is another object of the invention to significantly increase the yield of mariculture crops or catches such as oysters, cockles and other shellfish, and to facilitate the farming of corals for the sea aquarium trade in order to reduce the exploitation of natural reef resources.
While fish will hide behind any large sea bottom object, corals and most natural reef organisms will not settle and grow on and around conventional artificial reefs until many years or decades have passed. The reason appears to be that such reefs leach toxic chemicals and trace metals from cement, steel, plastics and paint for long after they are put in place, which inhibits the settling of corals and other organisms. Most of these traditional artificial reefs are biologically barren and do not produce genuine coral reef communities. Rather than organically becoming part of the environment, these underwater structures become dangerous projectiles in the event of hurricanes. After Hurricane Andrew hit Southern Florida, a survey of traditional articial reefs in the area revealed that not a single one remained intact. All moved, and while from one to many fragments were found of some, many vanished entirely.
The present inventors have found that on accreted artificial reefs and shore protection structures, an enormous variety of sea life is attracted, including young settling corals, juvenile fish, moray eels, sea urchins, sea cucumbers, crabs, squid, shrimps, bivalves, and even dolphins. The growth of corals on or near these structures is accelerated.